On a MIDI instrument channel strip with a drum instrument inserted, use the pencil tool to create a blank 'track'.
Stretch this MIDI track to fill 4 or 8 bars or whatever you're going to cycle.
Open the matrix editor for this MIDI track and use the pencil tool within the ME to add notes corresponding to the relevant drum parts.
Set the arrange window to cycle over the 4 or 8 bars of your MIDI region.
Press 'play' and let playback loop through the MIDI track as you build your fills.
[EDIT: just found this which may help http://flylib.com/books/en/18.104.22.168/1/ mainly the 'Exploring the Matrix Editor' section]
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